Uniform Packing Research Articles

Electrospinning Nonspinnable Sols to Ceramic Fibers and Springs.

Electrospinning has been applied to produce ceramic fibers using sol gel-based spinning solutions consisting of ceramic precursors, a solvent, and a polymer to control the viscosity of the solution. However, the addition of polymers to the spinning solution makes the process more complex, increases the processing time, and results in porous mechanically weak ceramic fibers. Herein, we develop a coelectrospinning technique, where a nonspinnable sol (<10 mPa s) consisting of only the ceramic precursor(s) and solvent(s) is encapsulated inside a polymeric shell, forming core-shell precursor fibers that are further calcined into ceramic fibers with reduced porosity, decreased surface defects, uniform crystal packing, and controlled diameters. We demonstrate the versatility of this method by applying it to a series of nonspinnable sols and creating high-quality ceramic fibers containing TiO2, ZrO2, SiO2, and Al2O3. The polycrystalline TiO2 fibers possess excellent flexibility and a high Young's modulus reaching 54.3 MPa, solving the extreme brittleness problem of the previously reported TiO2 fibers. The single-component ZrO2 fibers exhibit a Young's modulus and toughness of 130.5 MPa and 11.9 KJ/m3, respectively, significantly superior to the counterparts prepared by conventional sol-gel electrospinning. We also report the creation of ceramic fibers in micro- and nanospring morphologies and examine the formation mechanisms using thermomechanical simulations. The fiber assemblies constructed by the helical fibers exhibit a density-normalized toughness of 3.5-5 times that of the straight fibers due to improved fracture strain. This work expands the selection of the electrospinning solution and enables the development of ceramic fibers with more attractive properties.

Fixed-matrix regenerators optimization with wire mesh screens using entropy generation minimization method

The thermodynamic efficiency of a wire mesh fixed matrix regenerative heat exchanger mainly depends on the regenerator’s heating performance and losses. This paper aims to increase the effectiveness and decrease the pressure loss of static regenerators with wire mesh screens as packing geometry. The entropy generated for several combinations of mesh screens is calculated by using the entropy generation minimization method (EGM), and this method helps in finding the right combination with the minimum entropy generation. The study aims to minimize entropy generation by dividing the conventional uniform mesh regenerator pack length into multiple zones by accommodating different combinations of wire mesh screens. For the sample flow condition, it is observed that 3.94 × 103 W/K entropy is generated by uniform mesh, and from 68 hybrid mesh combinations, 80–120–200 generated 2.11 × 103 W/K. By balancing pressure loss and thermal efficiency, the potential of EGM-based optimization is underscored during this study, which improves the regenerator’s performance.

Spreading Behavior of Non-Spherical Particles with Reconstructed Shapes Using Discrete Element Method in Additive Manufacturing.

The spreading behavior of particles has a significant impact on the processing quality of additive manufacturing. Compared with spherical metal material, polymer particles are usually non-spherical in shape. However, the effects of particle shape and underlying mechanisms remain unclear. Here, the spreading process of particles with reconstructed shapes (non-spherical particles decomposed into several spherical shapes by stereo-lithography models) are simulated by integrating spherical particles with the discrete element method. The results show that more cavities form in the spreading beds of particles with reconstructed shapes than those of spheres with blade spreading. Correspondingly, particles with reconstructed shapes have lower packing densities, leading to more uniform packing patterns. Slow propagation speeds of velocity and angular velocity lead to "right-upwards" turning boundaries for particles with reconstructed shapes and "right-downwards" turning boundaries for spherical particles. Moreover, as the blade velocity increases, the packing density decreases. Our calculation results verify each other and are in good agreement with the experiment, providing more details of the behavior of non-spherical particles before additive manufacturing. The comprehensive comparison between polymer non-spherical particles and spherical particles helps develop a reasonable map for the appropriate choice of operating parameters in real processes.

Numerical investigation on the densification of granulated porous indium tin oxide powders before compaction

Dense and uniform initial packing structures of powders are of key significance in powder metallurgy process, which can provide the precondition for the fabrication of high-performance components with low cost and high efficiency. In this paper, the packing densification of indium tin oxide (ITO) granulated micro powders with continuous size distributions and internal porosity under three-dimensional vibrations was numerically investigated by discrete element method (DEM). Firstly, the DEM model was validated by physical experiments, and the contact parameters between particles and the porosity therein were determined. Then the effects of operating parameters on the macro/micro properties (e.g., packing density, coordination number (CN), pores, and forces) of the packing structures were analyzed. Results show that the mean CN cannot intuitively reflect the packing structure due to the complex and varied inter-particle contacts caused by the inhomogeneity of the particle size in a multi-sized granular system. Additionally, the excellent packing structure of porous ITO particles is always obtained from low amplitude when the vibration intensity is the same. Corresponding mechanism analyses reveal that higher amplitude normally causes strong convective diffusion, leading to more pores in the packing. This increases the probability of wedging effect occurring during settling, thereby reducing the packing density. However, optimal packing quality with low amplitude can only be achieved when coupled with high frequency. Therefore, applying vibration is an effective way to achieve dense packing of porous spherical particles, especially at low amplitude and high-frequency operating conditions.

Strain Rate and Temperature Influence on Micromechanisms of Plastic Deformation of Polyethylenes Investigated by Positron Annihilation Lifetime Spectroscopy.

Plastic deformation of low/high density polyethylene (LDPE/HDPE) was analyzed in this work using positron annihilation lifetime spectroscopy (PALS). It was shown that in undeformed LDPE, both the mean ortho-positronium lifetime (τ3) and its dispersion (σ3), corresponding to the average size and size distribution of the free-volume pores of the amorphous component, respectively, were clearly higher than in HDPE. This effect was induced by a lower and less uniform molecular packing of the amorphous regions in LDPE. During the deformation of LDPE, an increase in the τ3 value was observed within the local strains of 0-0.25. This effect was mainly stimulated by a positive relative increase in interlamellar distances due to the deformation of lamellar crystals oriented perpendicular (increased by 31.8%) and parallel (decreased by 10.1%) to the deformation directions. At the same time, the dimension of free-volume pores became more uniform, which was manifested by a decrease in the σ3 value. No significant effect of temperature or strain rate on the τ3 and σ3 values was observed during LDPE deformation. In turn, in the case of HDPE, with an increase in the strain rate/or a decrease in temperature, an intensification of the cavitation phenomenon could be observed with a simultaneous decrease in the τ3 value. This effect was caused by the lack of annihilation of ortho-positonium (o-Ps) along the longer axis of the highly anisotropic/ellipsoidal cavities. Therefore, this dimension was not detectable by the PALS technique. At the same time, the increase in the dimension of the shorter axis of the cavities was effectively limited by the thickness of amorphous layers. As the strain rate increased or the temperature decreased, the σ3 value during HDPE deformation increased. This change was correlated with the initiation and intensification of the cavitation phenomenon. Based on the mechanical response of samples with a similar yield stress, it was also proven that the susceptibility of the amorphous regions of LDPE to the formation of cavities is lower than in the case of amorphous component of HDPE.

Solid polymer electrolytes with LiNO3 as only lithium salt for highly stable lithium metal batteries

Lithium metal batteries (LMBs) have been gotten attention owing to high energy density. In here, the production of PEO-xLiNO3 solid electrolyte with LiNO3 as the sole lithium salt is cost-effective, environmentally friendly, and convenient. The interaction of the electrolyte and the electrode can form a uniform and dense solid-electrolyte interphase (SEI) layer packing L3iN and LiNxOy groups, which will effectively control lithium deposition and inhibit dendrite growth. The electrolyte exhibits outstanding mechanical properties, remarkable room temperature ionic conductivity, and the ability to suppress lithium dendrite growth. When applied in symmetric Li//Li batteries, the electrolyte shows excellent cycling stability with over 5000 h of cycling at a current density of 0.1 mA cm−2. For NCM622//Li batteries after full charge, it exhibits good cycling stability with a capacity retention rate is as high as 88 % after 300 cycles. Hence, the proposed electrolyte not only enhances safety performance but also improves electrochemical properties, making it highly promising for large-scale practical applications.

Synthesis and Characterization of a Novel Nanocomposite Polymer

One-dimensional nanostructures of PANI: PVA-g-EI nanocomposite are prepared by the interfacial polymerization method. The properties of a resulting green powder are studied by the X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), and infrared spectroscopy (FTIR). Prepared thin films were characterized by UV-Vis and photoluminescence (PL) spectroscopies. The XRD pattern of the nanocomposite shows that the higher volume fraction of crystalline phases corresponds to the PANI polymer with the accepted monoclinic unit cell of PVA. Nano-sized irregular particles arranged in clusters appear in the TEM measurements and SEM images, which testifies to the agglomeration without uniform packing. EDX confirms that the PVA-g-EI is incorporated in the structure of the polymer nanocomposite. A significant shift in the absorption edge with three PL independent emission peaks again confirms that PANI/PVA-g-EI form a nanocomposite.

Particle size and shape analyses for powder bed additive manufacturing

Technical advances in dynamic imaging have enabled routine sampling and analysis of particle shape and shape distributions. Size and shape distributions are relevant to many particulate processes involving flow, spreading, packing and densification. Powder bed additive manufacturing is a prime example requiring uniform spreading, packing and sintering of fine metal powders. This study focuses on quantitative representations of shape described in the International Standards Organization specifications and considers three aspects for improvement thereof: (1) reduced-order mapping of shape distributions using principal component analysis of shape descriptors; (2) uncertainty of shape distribution statistics based on pixel resolution; and (3) opportunities for analysis using machine learning for enhanced image resolution.

Smart gating membrane based on polyzwitterion-modified SiO2 nanoparticles for water treatment and in-situ multi-modal detection

The smart gating membranes with stimuli-response permeation variations have attracted tremendous interest in the field of wastewater treatment. Specifically, successful integration of accurate porous architecture, precise water permeance regulation, long-term duration and in-situ detection of multiple environmental signals in a visualized way is highly desired yet still exclusive. Herein, a smart gating membrane with a combination of accurate porous adjustment and multi-signals detection is developed by assembling poly[2-(1-(4-vinylbenzyl) pyridin-1-ium-4-yl) ethane-1-sulfonate] (PVPES)-grafted SiO2 nanoparticles on cellulose membrane. The membrane exhibits uniform packing architecture with highly tunable water permeance from 0 to 15,330 L m−2 h−1 bar−1, mediated via polymeric shell thickness control and external salt-stimulation; besides, in-situ detection of heavy metal ions and pH values of the treated solution is simultaneously achieved via the pH-induced discoloration and heavy metal ion-dependent fluorescence of PVPES. What’s more, the membrane shows great mechanical stability against repeated bending, vibration or abrasion, as well as great anti-fouling capability, which together contributing to a long service life. We believe the membrane developed here will definitely promote the practical application of smart gating membranes.

Dynamic monitoring of dielectric properties during two phase immiscible displacements in sand packs using frequency domain electromagnetic sweeps

Electromagnetic heating is a promising soil remediation method especially in thin formations. The lack of a wide-spread adoption of this method stems from insufficient knowledge of how the complex dielectric properties, that govern propagation of the electromagnetic waves through porous media, change with changing frequency, water saturation, displacement types and flow regimes. To breach these gaps several sets of spontaneous deionized (DI) water imbibition experiments, followed by the primary drainage floods, that were followed by the secondary DI water imbibition floods in confined uniform sand packs were performed. The frequency domain relative dielectric constant and conductivities were extracted from the two-port complex S-parameter measurements taken with the vector network analyzer during these immiscible displacements at various water saturation levels at ambient conditions. A novel coaxial transmission line core holder was designed and commissioned, and a modified version of a plane-invariant dielectric extraction algorithm was developed for this purpose. Series, parallel and semi-disperse mixing models were applied to fit the water saturation dependent relative dielectric constant and conductivity values sampled at 500 MHz from the extracted frequency domain spectra. The Maxwell-Garnett parallel model was proved to be the most flexible because it could capture the sampled conductivity values in all secondary imbibition floods before and after the breakthroughs, where the inflection points were observed. These inflection points were attributed to silica production and a potential shear-stripping flow. This observation was further confirmed by conducting a single-phase Darcy's law analysis of two DI water imbibition floods.

Siro false twist spun yarn structure and the knitted fabric performance

False twisting has been used to reduce yarn twists while keeping yarn tenacity, which is vital to improve fabric softness. In this paper, a false twisting device was integrated into a siro spinning system to develop a novel yarn structure with enhanced softness, in which the fibers were uniformly arranged. Siro false twist spun yarn was compared with a conventional siro spun yarn with the same count and twist in terms of yarn diameter, tenacity, hairiness, evenness, and packing density. The cross-sectional packing density of the yarns was analyzed based on micro X-ray computed tomography. The results showed that the siro false twist spun yarn has advantages in hairiness and evenness. The siro false twist spun yarn has an increased diameter by 10.4–19.0% and a more uniform cross-sectional packing density than the siro spun yarn. The two yarns were knitted into fabrics, and test results showed that the fabric produced with the siro false twist spun yarns had better compressibility, air permeability and light transmission. The findings from this study demonstrated that the siro false twist spun yarn improved fullness and its fabric has superior softness.

Nanomechanical characterization of 3D printed cement pastes

Cement-based 3D printing (additive manufacturing) has drawn significant attention in recent years as an emerging construction technology because of its potential environmental and economic benefits over the traditional cast-in-place concrete construction. Most research to date has focused on the macroscale properties of the printed structure. However, the performance of the printed materials depends on the properties of the individual filaments. In this study, grid nanoindentation coupled with scanning electron microscopy and energy dispersive X-ray spectroscopy was used to determine the effects of the printing process through extrusion on the local elastic indentation modulus and hardness of filaments in 3D printed cement paste structures. Dynamic changes in the water-to-cement ratio during the extrusion process combined with stress-induced dissolution of the cement particles led to variation in the median modulus of the printed filaments, with filaments having values greater than 23 GPa or lower than 20 GPa. Extrusion through a small diameter nozzle affected the mesoscale assemblage of the primary hydrate phases and led to printed filaments having a more uniform mesoscale packing of calcium silicate hydrates (C–S–H) with the disappearance of loosely packed C–S–H compared to their traditionally cast counterparts.

Numerical investigation of boiling process in micropores

The model for direct numerical simulation of the boiling process on porous surfaces was proposed and verified in the present paper. The VOF method using the CSF method to simulate surface tension was applied to solve this problem. The model was verified on the problem of pool boiling on a flat plate, where boiling patterns were obtained, and the heat transfer coefficient was compared with the experiment and analytical solution. Images of vapour bubble formation and dependence of heat transfer coefficient on temperature difference were obtained for a two-dimensional problem with uniform packing at various wall temperatures.

Alumina nanocrystalline ceramic by centrifugal casting

Centrifugal casting is an established molding method to prepare ceramics with high strength and high reliability and it has been well demonstrated in Al2O3. However, it has not yet been applied to Al2O3 nanocrystalline ceramic with < 100 nm grain size, primarily due to the unavailability of high-quality α-Al2O3 nanoparticles. In addition, ultrafine nanoparticles may be difficult to cast from the solution unless high-speed ultracentrifuge (e. g., >60,000 rpm) is used. Here we addressed these two challenges by home-made dispersed α-Al2O3 nanoparticles with 10 nm average particle size and HCl-assisted casting under a “normal” centrifuge condition and report the first attempt to produce Al2O3 nanocrystalline ceramic by centrifuge casting and pressureless sintering. The sintering kinetics and microstructure were analyzed, which assists the design of optimal two-step sintering schedule. We showed that dense Al2O3 nanocrystalline ceramic with 65 nm average grain size and ultra-uniform microstructure (the standard deviation of the grain size distribution to the average grain size is 0.358) can be obtained by two-step sintering at 1175 °C without holding followed by holding at 1025 °C for 20 h. The ultra-uniform microstructure may result from the denser and more uniform packing of particles in the green bodies produced by centrifugal casting. The two-step sintered Al2O3 nanocrystalline ceramic has a microhardness of 19.9 GPa. The microhardness indicates potential softening (inverse Hall-Petch relationship) of Al2O3 nanocrystalline ceramic at such a grain size.

Application of critical path analysis to streaming potential coupling coefficient in porous media

Streaming potential is a passive hydrogeophysical method that can be used to monitor water flow in porous media. However, a quantitative use of this method requires a good understanding of the streaming potential generation in water saturated porous media. In this work, we used the Critical Path Analysis (CPA) method to propose a new model to estimate the streaming potential coupling coefficient (SPCC) in porous media. By comparing the proposed model with 109 experimental data points for both unconsolidated samples such as uniform glass bead and sand packs and consolidated samples such as sandstones, carbonates and with 36 simulated data points, we demonstrated that the CPA is applicable to model the SPCC for porous media with both narrow and broad pore size distributions. Additionally, from the obtained result for the uniform grain packings, we found the rc–d relationship between the critical pore radius rc and grain diameter d that was validated by using 70 experimental data points available in literature. The obtained rc–d relationship may be helpful for applications of CPA-based models for uniform grain packings. We believe that the CPA-based model is an useful alternative approach for modeling the streaming potential coupling coefficient in porous media along with two other models available in literature.

Entropies of non-positively curved metric spaces

We show the equivalences of several notions of entropy, like a version of the topological entropy of the geodesic flow and the Minkowski dimension of the boundary, in metric spaces with convex geodesic bicombings satisfying a uniform packing condition. Similar estimates will be given in case of closed subsets of the boundary of Gromov-hyperbolic metric spaces with convex geodesic bicombings.

Graphene oxide-induced structural morphology and colloidal interaction at water–oil interface

Adsorption of amphiphilic monolayer graphene oxides (GOs) at oil–water interfaces can modify the interfacial structure and morphology, and in turn, influence their aggregating interactions and packing arrangement. A microscopic understanding of adsorption structures and interaction mechanisms for nanoscale GOs with varied chemical compositions at the interfaces can guide and improve the GO-related material preparation and micro-emulsification processes. Herein, all-atom molecular dynamics simulations were conducted for the structural morphologies and residing organizations of multiple GO nanosheets at the water-cyclohexane interface. The colloidal interaction was simulated as the potential of mean forces (PMF) between GO nanosheets at the interface. For the first time, the interfacial structural morphologies were revealed and correlated with the GO chemical compositions. GO nanosheets with more edge carboxyl groups, lower oxidation degrees, or lower hydroxyl proportion can lead to more uniform or homogeneous GOs packing, corresponding to weaker interface fluctuation or distortion. The PMF-based interactions can be applied to explain the GOs contacting performances at the interface, wherein the interfacial water plays a controlling role in regulating the GO interactions. Our simulation study provides new insights into the GO-induced liquid–liquid interface behavior.

Metastable Metal-Monolayer-Semiconductor Junctions: Diverse Chain-Length-Dependent and Ultraslow Electrical Progression

Molecularly tunable metal-semiconductor (MS) junctions have been fabricated by modifying mercury drop electrodes with n-alkanethiols, 1-CH3(CH2)n–1SH (n = 10, 11, 12, 14, 16, and 18) prior to the formation of intimate contact with hydrogen-terminated silicon (H-Si≡) and characterized by both solid-state electrical and electrochemical measurements. We have demonstrated that the current-voltage properties of these molecular junctions change with time and that diverse time-dependent progression “patterns” were observed between the systems of long-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkable that for mercury contact electrodes modified with long-chain alkanethiolate self-assembled monolayers (SAMs), the junctions became more rectifying and stabilized over a prolonged period (∼2 h). For short-chain counterparts, surprisingly, they initially became more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chain alkanethiolate SAMs first reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the other hand, achieve a more uniform, oriented, and stable packing as time goes by. These novel findings shed light on “long-term” intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated by simply varying the precursor structures (e.g., chain length of n-alkanethiols).

Investigation of liquid marble shell using X‐ray: shell thickness and effective surface tension

AbstractLiquid marble is a non‐wetting droplet encapsulated by micro‐ or nano‐sized hydrophobic particles. Recently, liquid marble has been emerging as a tool for digital microfluidics. Thus, a detailed understanding of the fundamentals of liquid marble is essential. The shell of a liquid marble has an opaque and fuzzy appearance which hinders in‐depth investigation using conventional optical microscopy. We used X‐ray computerized microtomography (CMT) to generate an image with a visible interface between the core liquid and the shell to overcome this problem. The interface facilitates accurate measurement of the shell thickness and the effective surface tension. This work investigates the effect of liquid marble preparation methods and liquid marble volumes on shell thickness and effective surface tension. We found that increasing the revolution speed during liquid marble preparation increases shell thickness. A liquid marble shell has a uniform packing when the revolution speed is 200–300 rpm. We also found that the effective surface tension of liquid marbles decreases with increasing volume. This could be due to a stronger effect of gravitational force for a large liquid marble. The findings from this work could provide a new insight into the characterization of liquid marble and open up a new direction of fundamental research of liquid marble shell.

Packing and doubling in metric spaces with curvature bounded above

We study locally compact, locally geodesically complete, locally CAT $$(\kappa )$$ spaces ( $$\hbox {GCBA}^\kappa $$ -spaces). We prove a Croke-type local volume estimate only depending on the dimension of these spaces. We show that a local doubling condition, with respect to the natural measure, implies pure-dimensionality. Then we consider $$\hbox {GCBA}^\kappa $$ -spaces satisfying a uniform packing condition at some fixed scale $$r_0$$ or a doubling condition at arbitrarily small scale, and prove several compactness results with respect to pointed Gromov–Hausdorff convergence. Finally, as a particular case, we study convergence and stability of $$M^\kappa $$ -complexes with bounded geometry.